On computer systems, specifying a query condition or calculation frequently requires selecting from a list of valid options to fill specific roles in a query condition or calculation. For example, conditions frequently take the form of <Variable><Operator><Variable/Value>, for example, “Sales >Costs.”
User interfaces frequently present such an equation using a series of drop-down list boxes or other controls to select from the valid list of choices to fill each of the positions in the equation. Alternatively, applications often allow users to edit equations by simply entering the information in a text entry box, which can be both difficult, since the user is not given any guidance, and very error prone.